There is a significant interest to develop benign processes for producing hydrogen that can be used as a fuel to power fuel cell vehicles. Such processes should reduce the amount of greenhouse gases produced, thus, minimizing impact on the environment. However, current methods for producing hydrogen incur a large environmental liability, because fossil fuels are burned to supply the energy to reform natural gas (primarily methane, CH4) to produce hydrogen (H2).
High temperatures above approximately 1500 K are required for producing hydrogen and carbon black at high rates by the direct thermal dissociation of methane [CH4+heat-->C+2H2] (reaction 1), ethane [C2H6+heat→2C+3H2] (reaction 2), propane [C3H8+heat-->3C+4H2] (reaction 3), or, in general, a mixture of gases such as natural gas generically represented as CxHy [CxHy+heat-->xC+(y/2)H2] (reaction 4).
Hydrogen can also be produced by the dry reforming of methane with carbon dioxide [CH4+CO2-->2CO+2 H2]. It is also possible to carry out dissociation of methane simultaneously with the dry reforming of methane if excess methane is present relative to that required to react carbon dioxide. Such processes are useful since they can provide for a high hydrogen content synthesis gas by utilizing natural gas from natural gas wells that contain a high concentration of carbon dioxide (typically 10 to 20 volume % CO2) or using landfill biogas (30 to 40 volume % CO2).
Hydrogen can also be produced by the thermal dissociation of hydrogen sulfide [H2S+heat-->H2+S] (reaction 5).
For these types of dissociation reactions, a solid (either C or S) is formed as a co-product (with H2) of the reaction. Often, the solid that is formed is in the state of fine particles. These particles have a tendency to deposit along the walls of reaction vessels or cooling chambers where the dissociation is occurring. If deposition occurs along the inside walls of the heated reactor, the particles tend to aggregate and crystallize. For the case of carbon deposition, the normally amorphous ultra-fine particles will grow in size and graphitize. Large graphitic carbon particles are less reactive compared to more amorphous fine sized particles and, hence, are of lower value. Furthermore, deposition on the reactor walls can cause plugging of the reactor and eventual shutdown of the process, thus, preventing continuous operation. In addition, carbon deposition on an outer transparent wall of a solar reactor can lead to overheating of the reactor wall.
U.S. Pat. No. 4,552,741, to Buck et al., reports carbon dioxide reforming of methane in a system comprising two catalytic reactors. One of the catalytic reactors is heatable with solar energy. In the abstract, the reactors are stated to be “filled with a catalyst”.
U.S. Pat. No. 5,647,877 reports solar energy gasification of solid carbonaceous material in a liquid dispersion. The solid carbonaceous material is heated by solar energy and transfers heat to a surrounding liquid. Hydrogen is produced in the process by the decomposition/gasification of the hydrocarbon (coal) particles.
EP 0675075A reports the use of solar energy to generate hydrogen from water. In the reported process, water is reduced to hydrogen with a metal, followed by reduction of the metal oxide with a reducing agent.
Hence, there is a need to develop high temperature environmentally benign processes for the production of H2 by thermal dissociation of hydrocarbon gases, such as natural gas, and to prevent the deposition of the products of dissociation on reactor walls.